diff --git a/fhirpathpy/engine/invocations/datetime.py b/fhirpathpy/engine/invocations/datetime.py
index 592ea7d..758d587 100644
--- a/fhirpathpy/engine/invocations/datetime.py
+++ b/fhirpathpy/engine/invocations/datetime.py
@@ -10,7 +10,7 @@ def now(ctx, data):
         if not now.tzinfo:
             now = now.astimezone()
         isoStr = now.isoformat()  # YYYY-MM-DDThh:mm:ss.ffffff+zz:zz
-        constants.now = FP_DateTime(isoStr)
+        constants.now = str(FP_DateTime(isoStr))
     return constants.now
 
 
@@ -18,7 +18,7 @@ def today(ctx, data):
     if not constants.today:
         now = datetime.now()
         isoStr = now.date().isoformat()  # YYYY-MM-DD
-        constants.today = FP_DateTime(isoStr).getDateTimeMatchStr()
+        constants.today = str(FP_DateTime(isoStr))
     return constants.today
 
 
@@ -26,5 +26,5 @@ def timeOfDay(ctx, data):
     if not constants.timeOfDay:
         now = datetime.now()
         isoStr = now.time().isoformat()  # hh:mm:ss.ffffff
-        constants.timeOfDay = FP_Time(isoStr)
+        constants.timeOfDay = str(FP_Time(isoStr))
     return constants.timeOfDay
diff --git a/pyproject.toml b/pyproject.toml
index ee610bb..b7851fc 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -55,7 +55,7 @@ classifiers = [
 requires-python = ">=3.8"
 
 [project.optional-dependencies]
-test = ["pytest==7.1.1", "freezegun==1.2.2", "pyyaml==5.4"]
+test = ["pytest==7.1.1", "pyyaml==5.4"]
 
 [project.urls]
 Homepage = "https://github.com/beda-software/fhirpath-py"
diff --git a/tests/test_evaluators.py b/tests/test_evaluators.py
index 4b53090..37a60dc 100644
--- a/tests/test_evaluators.py
+++ b/tests/test_evaluators.py
@@ -1,10 +1,10 @@
-import datetime
+import json
+from datetime import datetime, UTC
+from unittest import mock
 from decimal import Decimal
 import pytest
-from freezegun import freeze_time
 
 from fhirpathpy import evaluate
-from fhirpathpy.engine.invocations.constants import constants
 
 
 @pytest.mark.parametrize(
@@ -62,7 +62,11 @@ def simple_logic_expressions_test(resource, path, expected):
         ({"a": 3}, "a.exp()", [Decimal(3).exp()]),
         ({"a": 3}, "a.ln()", [Decimal(3).ln()]),
         ({"a": 3}, "a.log(3)", [Decimal(3).ln() / Decimal(3).ln()]),
-        ({"a": 3}, "a.truncate()", [Decimal(3).to_integral_value(rounding="ROUND_DOWN")]),
+        (
+            {"a": 3},
+            "a.truncate()",
+            [Decimal(3).to_integral_value(rounding="ROUND_DOWN")],
+        ),
     ],
 )
 def math_functions_test(resource, path, expected):
@@ -199,28 +203,44 @@ def misc_functions_test(resource, path, expected):
     assert evaluate(resource, path) == expected
 
 
-def time_functions_test():
-    local_tz = datetime.datetime.now(datetime.timezone.utc).astimezone().tzinfo
-
-    # tz_offset = datetime.timezone(datetime.timedelta(hours=2))
-    with freeze_time(
-        lambda: datetime.datetime(year=2020, month=8, day=20, hour=17, minute=52, second=15)
-    ):
-        assert (
-            datetime.datetime.fromisoformat(str(evaluate({}, "now()")[0])).timestamp()
-            == datetime.datetime.now().replace(tzinfo=local_tz).timestamp()
-        )
-        assert evaluate({}, "today()") == ["2020-08-20"]
-        assert evaluate({}, "timeOfDay()") == ["17:52:15"]
+@mock.patch("fhirpathpy.engine.invocations.datetime.datetime")
+@pytest.mark.parametrize(
+    ("expression", "expected_json"),
+    [
+        ("now()", '["2020-08-20T17:52:15.123+00:00"]'),
+        ("now() + 1 year", '["2021-08-20T17:52:15.123+00:00"]'),
+        ("now() + 1 month", '["2020-09-20T17:52:15.123+00:00"]'),
+        ("now() + 1 day", '["2020-08-21T17:52:15.123+00:00"]'),
+        ("now() + 1 hour", '["2020-08-20T18:52:15.123+00:00"]'),
+        ("now() + 1 minute", '["2020-08-20T17:53:15.123+00:00"]'),
+        ("now() + 1 second", '["2020-08-20T17:52:16.123+00:00"]'),
+        ("now().toString()", '["2020-08-20T17:52:15.123+00:00"]'),
+        ("timeOfDay()", '["17:52:15.123"]'),
+        ("today()", '["2020-08-20"]'),
+        ("today() + 1 day", '["2020-08-21"]'),
+    ],
+)
+def datetime_json_serialization_test(
+    datetime_mock: mock.MagicMock, expression: str, expected_json: str
+):
+    datetime_mock.now.return_value = datetime(
+        2020, 8, 20, 17, 52, 15, 123000, tzinfo=UTC
+    )
+    assert json.dumps(evaluate({}, expression)) == expected_json
 
 
-def now_function_test():
-    with freeze_time(lambda: datetime.datetime(2020, 8, 20)) as frozen_datetime:
-        old_now_value = evaluate({}, "now()")
-        frozen_datetime.tick(1.0)
-        new_now_value = evaluate({}, "now()")
+@mock.patch("fhirpathpy.engine.invocations.datetime.datetime")
+def now_function_test(datetime_mock: mock.MagicMock):
+    datetime_mock.now.side_effect = [
+        datetime(2020, 8, 20, 17, 52, 15, 123000, tzinfo=UTC),
+        datetime(2020, 8, 20, 17, 52, 16, 123000, tzinfo=UTC),
+    ]
+    old_now_value = evaluate({}, "now()")
+    new_now_value = evaluate({}, "now()")
 
     assert old_now_value != new_now_value
+    assert old_now_value == ["2020-08-20T17:52:15.123+00:00"]
+    assert new_now_value == ["2020-08-20T17:52:16.123+00:00"]
 
 
 @pytest.mark.parametrize(